This invention relates generally to processes for controlling the emissions of nitrogen dioxide and/or ammonia from geothermal power plants and is particularly concerned with a method of removing ammonia and ammonium ions from steam condensate produced in the power plant prior to passing the condensate to an open cooling tower.
General processes by which geothermal brine can be used to generate electric power have, of course, been known for some time. Geothermal brine, having a wellhead temperature of over about 400.degree. F. and a wellhead pressure of over about 400 psig, for example, can be flashed to a reduced pressure to convert some of the water in the brine into steam. Steam produced in this manner is generally used in conventional steam turbine-type power generators to produce electricity. After the steam passes through the turbine, it is normally condensed, and a portion of the condensate is passed to a cooling tower where it serves as make-up water. The "used" geothermal brine generated in the process is most commonly reinjected into the ground to replenish the aquifer from which it was extracted and to prevent ground subsidence. Reinjection of geothermal brine also helps to avoid the problems associated with the disposal of the large amounts of this saline and usually highly-contaminated liquid.
One of the many problems which has added to the overall cost of producing electric power by the use of geothermal brines relates to the undesirable emission of nitrogen dioxide (NO.sub.2) from cooling towers used in the power generating portion of the power plant. The nitrogen dioxide results from the oxidation of ammonia in the steam condensate to nitrites (NO.sub.2 .sup.-) by naturally occurring nitrifying bacteria, the subsequent reduction of the nitrites to nitric oxide (NO) by iron (Fe.sup.+2) with the concomitant production of hydrogen ions, and the oxidation of the nitric oxide to nitrogen dioxide upon contact with air in the cooling tower. Moreover, the formation of these hydrogen ions depresses the pH of the cooling tower waters which results in corrosion of the power plant's cooling system and piping used to dispose of unwanted condensate.
It has been taught in U.S. Pat. No. 5,032,284 that emissions of nitrogen dioxide from the cooling towers in geothermal power plants can be substantially avoided by contacting the steam condensate, usually in the cooling tower, with an amount of an oxidizing biocide which is effective to convert substantial amounts of the nitrites formed by oxidation of the ammonia into less reactive nitrate salts. A second nonoxidizing biocide (usually a carbamate) is added to the condensate in amounts and at periodic intervals which have been determined to effectively control the growth of organisms including nitrifying bacteria in the condensate handling system to below problem levels. Unfortunately, the cost of the oxidizing biocide and the carbamate biocide can be as much as $100,000 per year, and therefore this method of control is quite expensive.
As can be seen, there is still a need for new processes to control nitrogen dioxide emissions in the steam condensate handling portions of geothermal brine power plants in an economical manner.